Three steels. Three very different outcomes. Here’s how leading buyers spec them right—before the first cut.
If there’s one DFM question we get asked more than any other at SOLIDCO MOULD, it’s this:
“Why did my last mold start showing haze and pitting halfway through the run?”
Nine times out of ten, the answer traces back to one thing—the wrong steel grade for the resin and volume.
718H, NAK80, and S136 are the “big three” of plastic mold steels. They look similar on a quote sheet, but they behave nothing alike on the press. Mis-spec one, and you’re budgeting for rework before the tool even pays for itself.
Below is the exact decision matrix our engineering team applies to every new project.
1. Know the Trio: Property Snapshot
1.1 718H — The Versatile Workhorse
Pre-hardened 33–38 HRC, no heat treatment needed. Excellent machinability, forgiving on sharp ribs and deep cores, lowest material cost.
- ✅ Best for: Matte/textured shells, internal structural parts, complex geometries that fear cracking
- ❌ Limitation: Zero corrosion resistance; polish ceiling is low—mirror finish is off the table
- 📊 Typical life: 300K–500K shots
1.2 NAK80 — The Mirror Specialist
Pre-hardened 40–42 HRC, uniformly hard, ready to polish straight off the mill. The go-to for glossy, show-surface parts with short-to-mid lead times.
- ✅ Best for: High-gloss, non-corrosive cosmetic parts; fast turnaround (no heat treat delay)
- ❌ Limitation: Only mild rust resistance; acidic resins will stain it permanently
- 📊 Typical life: 500K–1,000K shots
1.3 S136 — The Corrosion-Proof Performer
Martensitic stainless, 48–52 HRC after hardening. Built-in rust immunity, acid-resistant, water-line safe, medical/food-compliant.
- ✅ Best for: Optical clarity, corrosive resins, million-shot programs, hygienic applications
- ❌ Limitation: Highest cost; thin sections and sharp corners need careful stress management
- 📊 Typical life: 800K–1,500K shots
2. Resin-Driven Selection: Don’t Guess
| Resin Family | Low/Medium Run | High-Gloss / Optical | Long-Run / Harsh |
| Neutral (PP, PE, std ABS) | 718H | NAK80 | S136H pre-hard |
| Transparent (PMMA, clear PC, light guides) | NAK80 | S136 (cleaner melt, zero pinholes) | S136 |
| Corrosive (PVC, POM, FR-grades) | ⛔ never 718H/NAK80 | ⛔ | S136 only |
| Glass-filled (PA+GF, GF-PC) | NAK80 (<15% GF) | Nitrided S136 | Nitrided S136 |
⚠️ Hard rule: Never run PVC or flame-retardant compounds in 718H or NAK80. Acid off-gas pits the cavity in just a few ten-thousand shots—polish can’t fix it.
3. Surface & Compliance Shortcuts
- Matte / textured / internal: → 718H (cost-first choice)
- Glossy cosmetic, short program: → NAK80
- Glossy cosmetic, long program: → S136
- Medical · Baby-care · Food-contact: → S136 mandatory (no exceptions for compliance)
4. Volume-Based Rule of Thumb
| Annual / Lifetime Volume | Recommended Steel |
| <300K shots (pilot / trial) | 718H |
| 300K–800K shots (mid-volume gloss) | NAK80 |
| 800K+ shots (long-run + harsh resin) | Hardened S136 |
5. Five Field-Tested Traps We See Every Quarter
- NAK80 + PVC/FR resin = irreversible oxidation. No regrind, no re-polish saves it.
- Thin-wall 3C molds in 718H drift dimensionally under sustained packing pressure.
- Tight deadline + corrosion need? Specify pre-hard S136H—skips heat-treat lead time.
- Sharp-corner sliders & inserts: keep slider steel in 718H; reserve S136 for cavity contact faces only.
- Smart hybrid builds: 718H/NAK80 cavity frames + S136 inserts at melt-contact zones. Cuts cost ~25% without sacrificing surface integrity. 💡
6. The Bottom Line
There is no “best” steel—only the best-matched steel.
718H for matte utility. NAK80 for fast-turn gloss. S136 for corrosion, optics, and million-shot endurance.
At SOLIDCO MOULD, steel selection is never a line-item guess. Every quotation ships with a written Steel–Resin–Volume compatibility note so you know exactly why we spec what we spec—before steel is even ordered.
📩 Starting a new tool? Let’s steel-review it together.
As your dedicated injection mold supplier and custom injection molding company, we engineer the details others overlook.
📩 [email protected] | 🌐 www.solidcomould.com | 📞 +86-18057673087
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